GSM is a set of telecommunication standards created by the European Telecommunications Standards Institute (ETSI) specifying the infrastructure of a digital cellular service. At the top level, the GSM system comprises a small number of networks that provide service around the world through interconnect agreements with each other. Access to the networks via sub-networks of service providers, which ultimately offer service to users, is usually accomplished through signaling brokers.
A GSM network includes a Home Location Register (HLR), which is a database of mobile subscriber information for a service provider's subscriber base. The information includes, for example, the International Mobile Subscriber Identity (IMSI), service subscription information, and location information, which includes the current Visitor Location Register (VLR) serving the subscriber, and service restriction and supplementary service information.
A VLR is a database that includes information about a mobile subscriber (user device) currently located in a given Mobile Switching Center (MSC) serving area when that serving area does not include the subscriber's HLR. Typically, there is one VLR per MSC, and the VLR and HLR exchange information regarding a user device. A network served by an MSC/VLR pair can be referred to as a mobile network. A mobile network may be, for example, ship-based such that the mobile network itself, as well as each of the user devices it serves, is mobile. A mobile network may instead be land-based with only the user devices being mobile.
In GSM, the Mobile Station International Subscriber Directory Number (MSISDN) is used to locate and route calls to a given user device. In addition, during each call setup phase, a user device is temporarily assigned a Mobile Station Roaming Number (MSRN) by its current MSC, which is used to help locate the user device when it receives a call by allowing the gateway system to route the call to the current serving MSC.
Traditionally, a service provider operates mobile networks that each require their own GT and set of MSRNs. However, the number of GTs and MSRNs available from a signaling broker is limited, which can limit the number of mobile networks a service provider can operate. Typically, a signaling broker buys a set of GTs and MSRNs from a GSM network and leases them to private GSM network providers. Thus, each additional GT and MSRN, even if available, represents an additional cost to the service provider.
The mobile networks may use a Very Small Aperture Terminal (VSAT) to relay voice data via satellite. For example, a ship-based mobile network may communicate via satellite. While the VSAT system offers a dedicated data channel with quality of service (QoS), or service and/or subscription based prioritization of traffic, VSAT equipment is expensive in relation to equipment for other Mobile Satellite Services (MSS) such as IRIDIUM and INMARSAT.
However, conventional use of MSS such as IRIDIUM and INMARSAT has also presented problems. Traditional implementations of GSM using MSS have used a Foreign Exchange Office (FXO) interface, which is a connector that allows an analog connection to the public switched telephone network (PSTN). For example, the Globe i250 from Globe Wireless, which included an INMARSAT FleetBroadband (FBB) terminal coupled with the MSC/VLR, also included an FXO interface to allow analog call transmission from a mobile network using the FBB to the PSTN.
The use of the FXO interface presents issues because the call is never routed back through the core GSM network throughout its duration. As a result, the service provider cannot easily and accurately track the duration of the call in order to properly bill the call based on usage. Additionally, the call recipient not within the mobile network (the recipient of a call placed from a ship using FBB, for example) sees the MSS number rather than the MSISDN of the calling user device. Related to this, user devices within the mobile terminal using FBB cannot cost-effectively receive calls.